121G 



TUNICATA. 



with the anal orifice (see Jig. 306. B,/, loc. cit.). 

 The sac is more compressed towards its lower 

 part ; and here are placed, externally to it, 

 the heart on the left, and the stomach and 

 the other viscera on the right side, the anus 

 opening upwards into the cloaca. On its sides 

 and front the sac is perforated by four rows 

 of narrow, vertical, irregularly oval holes or 

 spiracles, about sixteen in each row, placed at 

 less than the diameter of one of them apart 

 from each other. Through these the water, 

 which flows constantly in at the mouth when 

 its orifice is open, is apparently conveyed to the 

 vacant space between the sac and the mantle, 

 and it then escapes by the anal orifice. The 

 branchial tissue is extremely thin between the 

 stigmata, but their edges are thickened, and are 

 lined with closely set cilia, which by their 

 motion cause the current of water. When 

 these are in full activity, the effect upon the 

 eye is that of delicately toothed, oval wheels, 

 revolving continually in a direction ascending 

 on the right and descending on the left of 

 each oval, as viewed from without ; but the 

 cilia themselves are very much closer than 

 the apparent teeth, and the illusion seems to 

 be caused by a fanning motion made by them 

 in regular and quick succession, producing 

 the appearance of waves, and each wave an- 

 swering to the apparent tooth. The spaces 

 between the rows of spiracles are of much 

 more substance than the intervals between 

 the spiracles in a row ; filamentous processes 

 are stretched from them across the side cavi- 

 ties, attaching the branchial sac to the mantle : 

 these spaces also support finger-like processes, 

 about eight in a row, that project nearly at 

 right angles into the central cavity. 



The large short tubes of the branchial and 

 anal orifices have each five or six obscure 

 marginal indentations, and can be drawn in and 

 closed at the will of the animal. Within and 

 at the bottom of the branchial tube are nu- 

 merous, simple, tentacular filaments of different 

 lengths. The particles drawn into the bran- 

 chial sac by the current of water are seldom 

 stopped by these tentacles, but lodge some- 

 where on the branchial net-work. A lively 

 animalcule will sometimes disengage himself 

 by struggling, and dart about in the cavity 

 until he lodges on some other part ; or if a 

 morsel is found unsuitable, it is ejected by 

 the funnel's being closed, and the branchial 

 sac suddenly contracted vertically. Mostly, 

 however, whatever part the food lodges on, 

 it travels from thence horizontally with a 

 steady slow course towards the front of the 

 cavity, where it reaches a downward stream 

 of similar materials ; and they proceed to- 

 gether, receiving accessions from both sides, 

 and enter at last, at the bottom, the cesopha- 

 gus; which is a small flattened tube, carrying 

 them, without any effort of swallowing, towards 

 the stomach : the oesophagus takes a sharp 

 curve upwards and backwards before arriving 

 there. " It is extraordinary," observes Mr. Lis- 

 ter, " that the particles pass along so near to the 

 spiracles, with their cilia in full activity, with- 

 out being at all affected by them." " I have, 



says he, " in some positions, seemed to catch 

 a glimpse of a membrane suspended within, 

 too transparent to be commonly seen. One 

 may imagine the water to pass to the spiracles 

 strained through the meshes of such a mem- 

 brane, and the food to be carried along it by 

 invisible villi ; but this is mere conjecture." 



The stomach runs backward horizontally. 

 When seen from the side, its anterior portion 

 has an inflated appearance, and, when from 

 below, it seems to possess two lateral lobes. 

 The liver has an ochreous tint, and envelopes 

 the anterior portion of the stomach. The in- 

 testine on leaving the stomach rises, and then 

 bends forward with a sigmoid flexure, and ter- 

 minates in an ascending rectum and sphincter. 

 Transparent vessels ramify along a part of 

 the intestine, and meet at a collection of 

 globular bodies, from whence, in one indivi- 

 dual, two flattish lobes were observed to 

 extend backwards. These globular and lobate 

 bodies probably constitute the generative organs. 

 From the meeting of the above-mentioned 

 vessels two branches run ; one downwards 

 and backwards, but under the stomach, the 

 other forwards. From their direction Mr. Lis- 

 ter supposed them to communicate with a main 

 stream of blood near the heart. 



The circulation in these animals is very 

 interesting, and easily discerned through their 

 transparent tissues. The blood circulating in 

 one individual of the group descends by the 

 peduncle into the common root-like stem, and 

 penetrates into the next member of the group, 

 so that there exists in these Ascidians a com- 

 mon circulation, having as many centres and 

 motive organs as there are animals growing 

 on the same stem. The blood-globules are 

 very numerous, and though not uniform in 

 size or shape, are mostly between '00025 and 

 '0002 inch in diameter, and approaching to 

 globular. They are easily measured, as in the 

 intervals between the stigmata they pass mostly 

 but one at a time. The creeping tube, which 

 unites the individuals of a group, contains two 

 channels for two separate currents of blood, an 

 upward and a downward one, that are flowing 

 at the same time, and that send off each a 

 branch to every peduncle. The blood then 

 passes into the animal by one current, while 

 another carries it back. One of these canals 

 communicates, at the termination of the 

 peduncle, with the heart, which is placed, as 

 before mentioned, near the bottom of the 

 branchial sac on the left side, and consists of 

 a transparent ventricle or tube, running for- 

 ward and a little downward, in a channel 

 hollowed to contain it. Along the whole 

 length of this tube, a part on one side of its 

 axis seems fixed to the channel, the rest is 

 free and contractile. Mr. Lister observed that 

 when the blood entered the heart from the 

 peduncle, contraction began at the middle of 

 the ventricle, impelling onward the contents 

 of the forepart ; and the contraction of the 

 back part followed in the same direction, so 

 as for the whole to have the effect of one 

 pulsation. The heart was then filled again 

 by a flow from the peduncle. The intervals 



